This invention relates to a photoelectric detection apparatus with coaxial light emission and reception and more particularly to such a photoelectric detector for providing a low noise output signal.
Available photoelectric detection devices include a photosensor and a light emitting source both located on the axis of a lens for focusing received light on the photodetector. The light source is positioned to emit light in a direction forward of the lens, whereupon the emitted light may be reflected by an object passing in front of the lens. The reflected light is received by the lens and focused upon the photodetector for providing an output signal indicative of the presence of the object in front of the lens. Difficulty arises in interpreting the output signal as indicative of an object's presence due to the noise created in the output signal by spurious light from the light source which enters the lens directly from the light source. Internal reflection of light from the light source may also be transmitted to the photodetector further causing spurious output signal and masking the desired output signal. There is therefore, a need for a photoelectric sensing device which eleminates the major sources of noise in the output signal, and which thereby provides for a finer sensitivity to objects which are to be detected.